Electro-acoustic devices



Feb. 14, 1961 R. GORIKE 2,971,597 ELECTRO-ACOUSTIC DEVICES Filed Sept. 5, 1941 a U 2 T 1 IIYYENTOR. 1 1/001,] 00mm United States Patent ELECTRO-ACOUSTIC DEVICES Rudolf Giirike, Vienna, Austria, assignor to Henry Heinrich & Co.

Filed Sept. 5, 1941, Ser. No. 409,712

6 Claims. (Cl. 18131) The present invention relates to eiectro-acoustic devices and particularly to such devices for receiving and transmitting sound. More specifically, the invention relates to electro-acoustic devices the operative elements of which consist of a piston type diaphragm and an electro-mechanical system especially of the electrodynamic type connected with said diaphragm and either actuated thereby for receiving sound or itself actuating the diaphragm for transmitting sound.

An object of the present invention is to affect the resonance frequency of a piston type diaphragm so that all distorting reactions of the mechanical porperties of such diaphragm upon the reception or transmission of vibrations within the acoustical range will be suppressed or avoided.

Whereas earlier electro-acoustic devices used to be provided with so-called plate diaphragms as they are still in use in telephones and microphones of all commercial telephone systems, more recently sensitive devices of this type such as radio microphones, loudspeakers and the like, are provided with so-called piston type diaphragms, that is, diaphragms having a stiff active surface which are elastically suspended merely along their edge so as to vibrate as a unit similar to a piston.

Whereas in a plate diaphragm, due to the influence of higher frequencies, nodes of vibration always appear within the diaphragm surface resulting with such a diaphragm and the systems operatively connected thereto in very complicated frequency curves which can hardly be predetermined theoretically, for a diaphragm of the piston type resonance conditions may be obtained which are very accurately defined and the reactions of which upon the operatively connected electric and acoustic elements can be predetermined and calculated in every detail.

In order to make electro-acoustic devices having a piston type diaphragm sufliciently resistant against mechanical influences such as shocks especially in order to render them easily transportable, it is not only necessary to make the piston diaphragm as light as possible but also the stiffness of its suspending means must not be too small. Prior to the invention, it has therefore been impossible to place the mechanical natural vibration of a piston type diaphragm at the lower limit of the acoustic frequencies without incurring a very high senstivity to shocks. On the other hand, it would be useless to place the resonance frequency on the upper limit of the acoustic frequency range inasmuch as the system would then be too stiff and its efficiency too low. Every resonance frequency within the acoustic range leads, however, to distortion.

It has already been suggested to provide electro-acoustic devices of the type mentioned above at a point behind the piston type diaphragm with a closed slotlike air chamber of very shallow depth when seen in the direction of vibration. Such air chamber then forms behind the diaphragm an air cushion the elastic effect of which supports the mechanical stiffness of the diaphragm and therefore increases the natural frequency thereof. It is thus possible without any difficulty to raise the natural frequency of the piston type diaphragm to approximately 2000-6000 Hertz and to raise the frequency curve of the electroacoustic device within such frequency range accordingly.

In order to compensate the drop of the frequency curve within the range of the low frequencies, it has already been suggested to couple the mentioned shallow air chamber which is directly limited by the piston diaphragm, through a connecting slot with a rather undamped Helmholz resonator the natural frequency of which lies about one octave above the lowest frequency to be reproduced and the resonator neck of which terminating in front closely beside the piston type diaphragm contains a covibrating mass of air the acoustic impedances of which corresponds to the mechanical impedance of the piston diaphragm. The air in the resonator neck then vibrates within the range of the mentioned resonance frequency with opposite phases relative to the rear side of the diaphragm, that is, with equal phases relative to the front side thereof. Although in such a manner a sound transmitter or sound receiver, respectively, will be obtained the efiiciency of which is constant within a rather wide range of frequency, such devices can, however neither be combined with sound guiding means of the common type such as funnels or horns, not especially can they be used for producing a transformation of velocity, nor can they be coupled with sound transmitters or receivers of a different type for the purpose of obtaining any desired special distribution of sound pressure or sound sensitivity, respectively, without losing their tuning and thus their advantageous effects.

Another method known as such for obtaining a flat topped frequency curve by means of an electro-acoustic device having a piston type diaphragm and an air cushion, consists in coupling the mentioned cushion chamber with a highly damped resonance chamber the resonance frequency of which corresponds to the natural frequency resulting for the piston diaphragm from its mechanical impedance in cooperation with the stiffness of its suspending means and the elastic effect of the mentioned cushioning volume. Although it is hereby possible to suppress to a large extent the resonance peak caused by the intentionally increased natural vibration of the piston type diaphragm, at the same time a strong damping of the entire system will occur extending over a wide frequency range and affecting the efficiency considerably.

The present invention relates to an improvement of electro-acoustic devices having a piston type diaphragm, which improvement can be applied in accordance with the known proposals mentioned above both to sound transmitters as well as sound receivers, and which makes it possible for the first time to provide piston type diaphragms of low weight which in accordance with the requirements of transportation are equipped with sufficient mechanical stiffness, with a natural frequency which lies at, or even below, the lowest frequency of the acoustic frequency band to be governed. This apparently constitutes the ideal condition of an electro-acoustic device inasmuch as in this manner a constant of the efficiency comprising the entire frequency range of prac tical acoustics can be obtained without any undesirable damping, that is, without lowering of the entire level of efficiency and without affecting the possibility of coupling to such acoustic system sound guiding means of any desirable type or other auxiliary means for obtaining a certain distribution of sound pressure or sound sensitivity, respectively.

Another object of the present invention is to apply the known phenomena of acoustic transformation of velocity in diaphragm chambers having connected thereto a narrow sound guide in combination with the laws applicable to air columns vibrating in a longitudinal direction, in order to increase the mechanical impedance of the piston diaphragm by the acoustic impedance of an air column covibrating cophasely in longitudinal direction, in such a manner that the natural vibration of the system thus formed is decreased down to the lower limit of the practicalacoustic range, that is, to about 50 to 30 Hertz. Since covibrating air columns obviously do not aifect the mechanical resistance of a diaphragm toward shocks, the desired object will be obtained in this manner without affecting in any way the security against shocks.

These and other objects, features'and advantages of the present invention will be more fully understood from the following detailed description in connection with the accompanying drawings, in which:

Fig. 1 is a diagrammatic view of one form of the invention;

Fig. 2 shows diagrammatically an electric transmission circuit constituting an equivalent to the acoustical system shown in Fig. 1;

Fig. 3 shows in cross section another embodiment of.

the invention;

Fig. 4 is an electric circuit diagram equivalent to the device shown in Fig. 3;

Fig. 5 shows the frequency curve obtained with the device according to Fig. 3;

Fig. 6 shows in cross section another embodiment of the invention;

Fig. 7 is an electric circuit diagram equivalent to the device shown in Fig. 6;

Fig. 8 shows the frequency curve obtained with the device according to Fig. 6;

Fig. 9 shows in cross section still another embodiment of the invention;

Fig. 10 is an electric circuit diagram equivalent to the device shown in Fig. 9.

If, as shown in Fig. 1, a loudspeaker having a pistonshaped diaphragm M is provided with a funnel K the opening of which is smaller than the surface of the diaphragm M, at the opening 0 because of the transformation of velocity arising, variations of velocity 1; occur which are considerably higher than the variations p occurring at the diaphragm surface. If F is the vibrating surface of the piston type diaphragm M and F is thespeaker opening 0, the relation of these velocities is determined by the equation:

tion of velocity occurs also at this point, and the varia-' tions of velocity p arising at the front opening of the tube R M when the diaphragm M is vibrating, are determined in an according manner by the equation:

in which the ratio:

maybe called the ratio of transformation.

If merely the system is now considered which is formed by the diaphragm M and the elastic stiffness C of its elastic suspending zone as well as by the air chamber C and the tube M R it will be seen that when the diaphragm is vibrating, the mass of air in the tube R M is forced to execute longitudinal vibrations and forms anwimpedance which is operativelyconnected with the diaphragm M and thus lowers the natural vibration of the diaphragm. Although this mass as such is very small, the velocities to which it is subjected are in the same relation to the velocities to which the piston diaphragm is subjected, as the relation of the square of the surface F is to the square of the surface 1. Therefore, by means of the occurring transformation of velocity, this longitudinal covibrating mass of air acts as a mechanical mass which is connected to the diaphragm by means of the longer arm of a two-arm lever, the length of the arms of which being according to the Equation 1: F :f =T. Thus, the acoustic impedance M of this air column covibrating in longitudinal direction is determined by the specific weight s of the air, and the length l and the cross sectional area f of the tube R M according to the formula:

introduced into the acoustical consideration of the system disclosed can best be seen from the theoretically equivalent electrical diagram shown in Fig. 2. The impedance M of the piston diaphragm first forms together with the stiffness C of its suspending means and the stiifnessC of the air cushion behind the diaphragm an oscillatory operating circuit which is shown in the diagram according to Fig. 2 as an oscillatory circuit C, M, C E. The vibrations of the piston type diaphragm produced by electric or acoustic driving energy, respectively, act upon this oscillatory circuit as an impressed alternating potential E. The mentioned oscil latory circuit is operatively connected through a resistance R indicating the frictional resistance of the air Within the tube R M with the impedance M of the air column longitudinally vibrating within the tube, whereby said impedance is to be considered according to the Equation 2. Thus, a second oscillatory operating circuit C, M, R M E is formed.

Tests have shown that the resonance frequency of the oscillatory circuit M, C, R M E of piston type diaphragms the mechanical natural vibration of which lies at about to 200 Hertz and which thus have at least some resistance to knocking and shocks, can be brought down without difficulty to 5030 Hertz. It is therefore possible to obtain in this manner at the lower region of the acoustic frequency range a resonance resulting in a compensation of the frequency curve as this could previously only be obtained by the above mentioned means which all disclosed basic disadvantages of various kind.

Obviously, for this purpose it assumed that the capacity C according to Fig. 2, that is, the air cushion C behind the diaphragm M according to Fig. 1, will be made sufficiently small so as not to act as a capacity short circuit which would decouple the impedance M; from the oscillatory system C, M, C E. The easiest way of obtaining this is by making C so shallow that the natural frequency of the oscillatory circuit M, C, C E is placed at the upper region of the frequency range to be governed, that is, for example, within the region of 8,000 to 10,000 Hertz.

If, according to Fig. 1, the velocity transforming chamber in front of the diaphragm M and the funnel K are removed, the resulting arrangement, similar to every rearwardly open system, forms a pressure gradient receiver or transmitter, respectively, which acts similar to a band microphone or band loudspeaker, but can be equipped with a normal magnetic, dynamic or capacitative driving system so that it does not require any special adapter transformer in order to combine this system with an. acoustic pressure receiver or transmitter of the usual kind or of the kind according to the invention, for obtaining receiving or transmitting characteristics of any special type. If, on the other hand, the side of thetube R M,

directed away from the diaphragm terminates into a chamber which is closed to the outside, for example, by the housing of the entire system, the inner space of such housing acts as a capacity C lying in series with the impedance M as indicated in dotted lines in Fig. 2.

Concerning the tube R M the above Equation 2 shows that only the cross sectional area and the volume enclosed thereby are of importance. Therefore, this tube may also be given the shape of, for example, an annular slot which may at the same time be utilized for accommodating the vibrating coil of the dynamic driving system. However, it has been found to be of greater advantage to utilize said annular slot for coupling a further stiffness element to the system and hereby to form still another suitably tuned oscillatory operative circuit. It has been found that in such a manner further surprising ajcflvantages may be obtained, as will be described herea ter.

A close study of the diagram according to Fig. 2 shows that aside from the two oscillatory circuits C, M, C E and C, M, C R M E which directly include the source of alternating current E and which may therefore be called operative circuits, it also contains the resonance circuit C R M which is coupled to the source of alternating current B through the alternating current resistance M, C and therefore acts as an energy dissipating circuit as it destroys the energy which it withdraws from the driving system. As long as the'impedance M is not more than ten times as high as the impedance M, the resonance frequency of this coupled energy dissipating circuit can be held above the frequency range to be reproduced, that is, for example, at 10,000 Hertz. However, since the known oscillation formula of the natural frequency varies only in proportion to the square root of the impedance, this means that the mechanical natural vibration of the diaphragm M can be brought down by the impedance M only to about a third of its mechanical value, so that the mechanical natural frequency of the diaphragm M may only be placed at about 150 Hertz if in the higher ranges of the frequency band to be transmitted undesirable saddles in the frequency curve caused by said energy dissipating circuit should be avoided.

If a further stiffness element is coupled to the system by means of the annular slot provided for accommodating the vibrating coil, such saddles may be avoided by coup ing a further operative resonance circuit to the cushion chamber. The impedance M can then safely be given the value which mechanically is the most suitable. Since piston type diaphragms with an attached vibrating coil are made with the least difliculty and with the most favorable combination of shock resistance and electro-mechanical efliciency for a mechanical natural vibration of about 400 to 500 Hertz, this value of M is about a hundred to four hundred that of M, that is, a value which according to the invention resu ts in lowering the acoustically active natural vibration ten to twenty times relative to the original mechanical natural vibration.

An embodiment of a pressure receiver constructed in such a manner according to the invention is shown in Fig. 3. The diaphragm M having the usual dome shape, carries at its edge the vibration coil S within the ring shaped elastic suspension Zone C. The vibrating coil S enters into an annular s ot R behind which a volume of air C is provided in a known manner. The cushion chamber C provided asusual behind the diaphragm and limited by the pole piece B of suitable shape communicates according to the invention with a small tube R M the enclosed volume and the opening of which facing the diaphragm are thus tuned to the mechanical impedance of the diaphragm that, in view of the transformation of velocity arising, the impedance M of the air column covibrating in longitudinal direction within the tube R M; reacts upon the natural frequency of the piston type diaphragm with a multiple T of the mechanical impedance M of said diaphragm. For this purpose, the relation of the acoustically active impedanceM to the impedance M is made so extremely large that the natural frequency of the oscillatory system which is formed by the cushioning volume C and the covibrating volume R M enters into the range of the frequency band to be governed. However, at the same time, the natural frequency of the oscillatory system which is formed by the impedance M of the diaphragm, the stiffness C of the diaphragm suspension, the stitfness of the air space C behind the annular slot R and the impedance M is tuned approximately to this same frequency, the impedance-M which may sometimes be negligible being calculated for the volume of air covibrating in longitudinal direction in the annular slot R in consideration of the transformation of velocity arising.

The importance of these operations will easily be seen from Fig. 4 showing the theoretically equivalent electrical diagram for the system according to Fig. 3. This diagram generally shows three operative circuits directly supplied by the source of alternating current E, and two energy dissipating circuits coupled thereto, namely:

(1) The operative circuit M, C, C E the natural frequency of which should lie at the upper region of the frequency band to be governed;

(II) the operative circuit M, C, R M C E the natural frequency of which should lie at the lower region of the range of frequency to be governed;

(III) the energy dissipating circuit R M C C the natural frequency of which should lie either above the upper region of the range of frequency to be governed or which may be placed at any region of the range of the frequency to be governed if it is made substantially equal to the natural frequency of-- (IV) the operative circuit C, M, R M C E which, with suitable damping 2), then overcomes the detrimental effects of the energy dissipating circuit mentioned under (III);

(V) the energy dissipating circuit R M C C M R the resonance frequency of which necessarily lies closely to the resonance frequency of the oscillatory cir cuit (II) as it contains the same elements, which, however, does not come into acoustic appearance since it contains the resistances R and R in series and therefore being extremely damped.

That these conditions can actually be applied in practice may be seen by the following example:

Experiments have proven that a piston type diaphragm having an effective surface area of 7 cm. and a weight M, including that of the vibrating coil, of 0.1 gram can be built without any difficulty. The stiffness of the diaphragm suspension has been measured to be 44-10 Dyn./cm. whereas for the annular slot R provided behind the vibrating coil a frictional resistance was determined amounting with regard to the diaphragm M to about 2000 ohms.

With such a diaphragm the mechanical natural vibration of which lies at about 400 Hertz, the best results have been found to be if a cushion C having a volume of 0.35 orn. is used in combination with a small tube R M having a length of 1.3 cm. and a cross sectional area of 0.01 cm. a housing C having a volume of 250 cm. and a resonance C having a volume of 30.6 cm. which is coupled to the cushioning volume through the annular slot R The above values and the formulae mentioned'in the beginning 'then lead to the following values:

M :01 gram C =4.4-10 Dyn./cm.

M =9.8 grams C =2.3- l0 Dyn./cm.

C =2.8- 10 Dyn./cm.

C =2.3 -10 Dyn./cm. R =2000 ohms On the basis of the well known oscillation formula:

into which for calculating each circuit the sum of the R =1200 ohms stitr'ness'elements (C+C and the sum of the impedance elements (M+M contained in this circuit are to be inserted, thefollowing values will be found forthe individual oscillatory systems mentioned above:

(I) V =8000 Hertz (II) V =45Hertz (III) V =800 Hertz (IV) V =800 Hertz The influence of the resistances R and R upon the resonance frequency is negligible. However, it is important that the resistance R be made relatively large in order to. damp the operative circuit IV in a suitable manner. Sincean acoustic frictional resistance is proportional to the circumference divided by the surface area of a given'channel cross section and slotlike channels therefore result in especially high frictional resistances, the annular slot R fulfills this requirement in a very suitable manner. If the annular slot R should not be used for coupling the resonance volume C to the cushioning volume, a larger number of narrow channels arranged parallel to one another like a sieve will best be suitable for this purpose.

The measured frequency curve of the microphone the values of which have just been discussed in detail, is shown in Fig. 5 in a full line, whereas the frequency curve arising when the front opening of the tube M 11 is closed, is indicated in this figure in dotted lines. It

will be seen that the most important difference between these curves lies in the fact that according to the invention, the drop of efficiency below approximately'600 Hertz will be avoided and that in this manner a practically straight frequency curve may be obtained extending from 30 Hertz to approximately 10,000 Hertz.

In devices the range of frequency of which only needs to extend to about 8,000 Hertz, the very small impedance M cooperating with the annular slot R may usually be neglected. If, however, a range of frequency up to more than 10,000 Hertz should be safely covered, a circuit which so far has not been considered, will be found to have a disturbing influence, namely:

(VI) The energy dissipating circuit C R M C the natural frequency of which is relatively high because of the very small impedance M usuallylying at about 12,000 Hertz. In order to compensate the effect of this circuit, it is possible to provide in an analogous manner to the provision of the resonance circuit IV for compensating the energy dissipating circuit III, a further resonance circuit R M C as shown in Figs. 6 and 7, which consists of an air chamber C and a channel R M connecting said air chamber with the cushioning chamber C the' chamber C then being suitably provided at the inside of the pole piece B, as shown in Fig. 6. A frequency curve obtained with this system is shown in Fig. 8.

Obviously, the same principle may be further applied several times in order to avoid even the least irregularities in the frequency curve shown in Fig. 8. However, diificulties will then easily arise regarding the space for providing the various channels and air chambers, inasmuch as the length of the channels may not exceed one half of the wave length of the highest frequency of the frequency band to be covered in order that stationary waves will not form at the inside of such channels.

These difficulties as to the necessary space may, however, be avoided according to the invention as shown in Fig. 9, by providing these channels, the length of which is in the danger of exceeding one half of the shortest wave length of the frequency band to be covered, at suitable distances from one another with lateral apertures terminating in separate air chambers 'Ca-Cd. A channel thus composed of the sections MaMe then forms an acoustic filter as indicated in Fig. 10, in which the sections Ma-Me lying between the individual apertures act like self-inductances connected in series between which capacities (Ia-Cd are connected. Obviously, such. construction fully avoids the formation of stationary" waves. It is, however, only necessary to apply this tea-- ture merely to channels the length of which exceeds'the" amount of about 1.3-1.5 cm., inasmuch as, for example, a channel length' of 1.3 cm. is already sufficiently short to cover an acoustic frequency band extending up to 1. An electro-acoustic sound transducing device for transmitting a predetermined frequency range, compris-- ing a diaphragm of the piston type, wall means cooperat-- ing with the rear side of said diaphragm to define an air cushion in back of the latter, wall means defining an air channel communicating, at one end, with said air cushion and opening, at the other end, into a relatively large volume of air, the air'in saidohannel being directly coupled to said diaphragm by Way of said air cushion to vibrate in phase with said diaphragm, the length of said channel being less than one half the wavelength of the highest frequency of said range and being sufficiently large in relation to the cross-sectional area of the channel as to cause a longitudinal vibration of the air in said channel in response to vibration of said diaphragm, said channel being dimensioned to contain a mass of air defining an acoustic impedance which is at least ten times as large as the mechanical impedance of said diaphragm so that said mass of air and said diaphragm constitute a vibrating system having a lower natural frequency than the diaphragm alone.

2. An electro-acoustic sound transducing device as in claim 1; wherein said channel has a length approximately one hundred times as large as said cross-sectional area of the channel.

3. An electro-acoustic sound transducing device as in claim 2; wherein said air cushion has a depth equal to approximately seven-thousandths of the effective area of said diaphragm.

4. An electro-acoustic sound transducing device as in claim 1; wherein said relatively large volume of air is contained in means defining a. first chamber communicating with said air cushion by way of said channel; and further comprising means defining a second chamber, and frictional resistance means through which said second chamber is coupled to said air cushion to damp the resonance of said vibrating system, said mass of air in said channel defining an acoustic impedance having a value approximately one hundred to four hundred times as large as the mechanical impedance of said diaphragm.

5. An electro-acoustic sound transducing device as in claim 4; wherein said diaphragm has a weight of 0.1 gram and an effective surface area of 7 cm. said air cushion has a volume of 0.35 cm. said channel has a length of 1.3 cm. and a cross-sectional area of 0.01 0111. said first and second chambers have volumes of 250 cm. and 30.6 cm. respectively, and said frictional resistance means have a value of 2000 ohms.

6. An electro-acoustic sound transducing device as in claim 4; further comprising a Helmholz resonator tuned to the upper part of said frequency range and coupled to said air cushion.

References fitted in the file of this patent UNITED STATES PATENTS 

